Authorities said at least four people were killed when a stage collapsed Saturday night during a storm at the Indiana State Fair, where the country group Sugarland was set to perform.

About 40 people were reported hospitalized in the incident at the fairgrounds in Indianapolis, state police said. Strong winds caused the stage rigging for the outdoor concert to collapse, trapping and injuring concertgoers shortly before 9 p.m.

No one was performing at the time. The opening act had finished, and the crowd was waiting for Sugarland to take the stage.

State police said that those hospitalized had suffered a range of injuries from minor to critical. They said search and rescue operations at the site had been completed.

Thousands of people were being evacuated from the fairgrounds grandstand to a nearby coliseum because of worsening weather when a wind gust brought down the stage rigging. It fell onto an area where some fans were seated.

A YouTube video uploaded moments after the collapse showed the stage tumbling forward on top of fans and the chaos afterward. "Oh my God, Oh my God," one woman can be heard saying.

"It was like it was in slow motion," concertgoer Amy Weathers told the Indianapolis Star. "You couldn't believe it was actually happening."

The injured were being moved to a tunnel below the stage, the newspaper reported.

﻿﻿﻿﻿Shortly after the accident, Sugarland said on Twitter: "We are all right. We are praying for our fans, and the people of Indianapolis. We hope you will join us. They need our strength."

Associated Press photographer Darron Cummings was in the audience attending the concert as a fan shortly before the collapse. He said an announcer gave the crowd instructions on how to evacuate if the weather worsened but said they hoped to get Sugarland on stage soon.

Cummings said he and his friends sought shelter in a nearby barn after seeing the weather radar.

"Then we heard screams. We heard people just come running," Cummings said.